The present embodiments relate to a method and a control sequence determination device for the determination of a magnetic resonance system activation sequence.
In a magnetic resonance tomography system (e.g., magnetic resonance system), the body that is to be examined may be subjected to a relatively high main magnetic field (e.g., the B0 field; 3 or 7 Tesla), with the aid of a main field magnetic system. In addition, a magnetic field gradient is applied with the aid of a gradient system. Using a high-frequency transmitter system, high-frequency excitation signals (HF signals) are emitted by suitable antenna devices, the intention of which is that the nuclear spins of specific atoms or molecules, excited to resonance by this high-frequency field, are tilted by a defined flip angle in relation to the magnetic field lines of the main magnetic field. This high-frequency excitation and, respectively, the resultant flip angle distribution are also designated hereinafter as nuclear magnetization or simply “magnetization”. With the relaxation of the nuclear spin, high-frequency signals (e.g., magnetic resonance signals) are irradiated. The high-frequency signals are received by suitable reception antennae and then undergo further processing. From the raw data acquired in this way, the desired image data may be reconstructed. The transmission of the high-frequency signals (e.g., the B1-field) for the nuclear spin magnetization takes place by a “whole body coil” arranged permanently in the device around the measuring area (e.g., patient tunnel). Reception of the magnetic resonance signals takes place in most cases with the aid of local coils that are positioned more densely on the body of the patient. Reception of magnetic resonance signals may also be carried out with the whole body coil, and/or the transmission of the HF signals may be carried out with the local coils.
For a specific measurement, an activation sequence with a high-frequency pulse sequence to be transmitted, and a gradient pulse sequence that is to be switched in co-ordination with this (e.g., with suitable gradient pulses in the layer selection direction, in the phase coding direction, and in the read-out direction, frequently in the z-direction, y-direction, and z-direction), and further control specifications are defined in a measurement protocol. This measurement protocol may be established in advance and be called up for a specific measurement (e.g., from a memory) and, if appropriate, may be changed on site by the operator. During the measurement, the control of the magnetic resonance system may be carried out fully automatically on the basis of this activation sequence, where the control device of the magnetic resonance system reads out the commands from the measurement protocol and processes the commands.
For the generation of the activation sequence, in, for example, an optimization process, the individual HF pulse sequences (e.g., the HF trajectories) are determined for the individual transmission channels over time as a function of a fixed “k-space trajectory” that may be specified by a measurement protocol or individually by an operator. The “transmission k-space trajectory” (e.g., a “trajectory”) relates to the locations in the k-space that are moved to by the adjustment of the individual gradients at specific times. The k-space is the spatial frequency space, and the trajectory in the k-space describes by which route the k-space will be run through temporally when an HF pulse is transmitted, by appropriate switching of the gradient pulses. By adjusting the k-space trajectory, at which spatial frequencies specific HF energy quantities will be deposited may be determined.
For the generation of the activation sequences in this situation, account may additionally be taken in the optimization process of currently measured B1-maps that, in each case, indicate the spatial B1 field distribution for a specific antenna element, and a B0 map that spatially resolves the off-resonances or represents the deviation of the B0 field from the homogeneous B0 field that is actually desired (e.g., the Larmor frequency that is actually being striven for). In addition, for the planning of the HF pulse sequence, the user may specify a target magnetization such as a desired flip angle distribution. With a suitable HF pulse optimization program, the suitable HF pulse sequence is calculated, such that the target magnetization is attained. This may involve the most homogeneous possible magnetization in the Field of View (FoV) that is to be examined, or, respectively, the Field of Excitation (FoE) that is to be excited.
With more recent methods, quite specific regions (e.g., two-dimensional) may be excited within a layer (e.g., a non-homogeneous target magnetization is deliberately striven for).
One possibility of determining a two-dimensional high-frequency pulse sequence (e.g., a “2DRF-Pulse”) in the manner described heretofore is described in the article “Magnitude Least Square Optimization for Parallel Radio Frequency Excitation Design Demonstrated at 7 Tesla With Eight Channels” by K. Setsompop et al., Magn. Reson. Med. 59: 908 to 915, 2008. In this situation, the transversal target magnetization is represented in a linear matrix equation system from the spatial coil profiles and the multi-channel high-frequency pulse sequences, into which information is also introduced about the B0 maps and B1 maps present and the k-space trajectory used. This equation system is resolved numerically for a specific predetermined target magnetization in order to obtain the appropriate high-frequency pulse sequence.
In relation to the trajectories that may be used, however, with constant gradients that are used for a simple layer-selective excitation, such one-dimensional, two-dimensional, or multi-dimensional k-space trajectories for selective excitation exhibit a substantially greater complexity. Due to this greater complexity, there is also an increased risk of artifact formation in the images (e.g., because such pulses may be substantially longer). Complicated k-space trajectories of this type may therefore be determined within the framework of an optimization process automatically, taking account of specific stipulations (e.g., jointly with the HF pulse sequence).